The voltage dependence of the current-induced FL torque acting on the free layer is displayed in Fig. REF (b) for several asymmetry parameters \(\delta \) . First, one notices that the interlayer exchange coupling (IEC), being the magnitude of this torque at equilibrium, i.e., at \(V=0\)  V, changes as one tune the asymmetry of the MTJ. This feature, that is apparent by the shift in the horizontal dashed lines, can be understood in terms of Bruno's model for IEC [1]}, where its strength depends on the relative value of the reflection coefficients of majority and minority spins at both oxide/ferromagnet interfaces. Hence, as \(\delta \)  increases, electrons scatter differently as they encounter a larger energy barrier at the B interface, modifying the strength of the IEC between fixed and free layer mediated by the insulating spacer. Secondly, while the DL torque always presents qualitatively similar voltage dependencies with varying \(\delta \) , the FL torque might completely change its voltage dependence from purely quadratic (\(\delta = 0\)  eV) to linear (see, for example, the \(\delta = 1.8\)  eV case). Such features were predicted in the context of a similar TB approach and used to explain the FL torque voltage dependencies observed by different experimental groups [2]}, [3]}. Such linear-in-voltage behavior of the FL torque was also predicted to be useful in assisting bipolar magnetization switching in asymmetric MTJs having non-negligible voltage control of magnetic anisotropy (VCMA) in the free layer [4]}.
